The electrochemical production of hydrogen is performed by alkaline electrolysis. A byproduct is oxygen. When the hydrogen exits the electrolyser, aerosols (small droplets) of the electrolyte, which is usually a solution of potassium hydroxide, are carried over with the gas stream. The concentration of the potassium hydroxide in the hydrogen usually is between 3.0 mg/m3 and 5.0 mg/m3 depending on the structure of the electrolyser and applied electrolysis conditions. A description of the electrolytical production of hydrogen is given in Smolinka, T: “Wasserstoff aus Elektrolyse—ein technologischer Vergleich der alkalischen and PEM-Wasserelektrolyse”, FVS Workshop, 2007.
It is very important to remove the potassium hydroxide from the product gas in order to avoid problems in the subsequent process steps. Potassium hydroxide concentrations (>1.0 mg/m3) may cause corrosion on metal elements. This is in particular relevant for subsequent compressors necessary to provide the pressure required for a downstream process as e.g. the methanation process or the direct feed of hydrogen gas into methane gas distribution system. In addition, catalysts may be deactivated and the sealing properties of pipe parts such as valves, etc. may be impaired by crystallization of the potassium hydroxide. Therefore, hydrogen gas that contains droplets of potassium hydroxide aerosols is not suitable for further use.
It is, therefore, known to remove the potassium hydroxide from the product gas by means of a gas scrubber usually located between the electrolyser and the compressor prior to a further use. The removal of the potassium hydroxide is positively influenced by introducing demineralized water which is enriched with the potassium hydroxide and thus can be recycled to the electrolysis process as feed water.
In order to provide for a safe operation, there must be an excess pressure on the suction side of the compressor as compared to the ambient pressure of at least 0.5 kPa gauge. It must be prevented that during the use of the compressor a vacuum is created on the suction side which in the case of leakage in the system will allow air (oxygen) to enter. This may create an explosive gas mixture.
The usually applied gas scrubbers in combination with a low-pressure electrolyser, however, have the following problems: Either, the internal pressure loss of the apparatus is too high so that—depending on the pressure in the electrolysis step—the required excess pressure cannot be maintained. In other types of the gas scrubber the separation efficiency with regard to potassium hydroxide is not high enough to provide the required purity of less than 1.0 mg/m3. Relevant for the separation efficiency with regard to potassium hydroxide aerosols is the size of the droplets. Common gas scrubbers can remove potassium hydroxide droplets above a size of about 20.0 μm (diameter) with a low pressure loss. The major part of the droplets having a diameter of less than 20.0 μm that is typical for aerosols, can only be removed by means of a device causing a high pressure drop, such as a venturi scrubber or a fixed bed packing. Such packings, however, involve high pressure losses and are quite expensive.
A further contaminant of the hydrogen gas stream is oxygen, which is the second gas product of water electrolysis. This contamination is created by incompletely gastight diaphragms between the anode and the cathode of the electrolyser. The oxygen content in the hydrogen gas is, dependent on the operation state, between 0.0 and 2.0 vol.-%, corresponding to about 50.0% of the lower explosion limit. Values above 2.0 vol.-% are not admissible in the system and will consequently lead to an automated safety shutdown of the electrolysers. Elevated oxygen concentrations in the hydrogen gas reduce gas purity and may also be detrimental to catalysts if the hydrogen is used in subsequent processes. Usually, the oxygen is removed after the scrubbing step by catalytic oxidation converting it with hydrogen to water vapor.
The final step of the hydrogen gas treatment either for the use in a downstream process or for direct feed of hydrogen gas into methane gas distribution system is the gas drying. Usually, adsorption dryers filled with a drying agent like silicagel are used to remove the remaining moisture from the hydrogen. Usually two drying units are provided in parallel and operated one at a time to ensure a continuous operation. One unit is used while the other unit is regenerated to remove the adsorbed water vapor.